1. Field of the Invention
The present invention relates to an optical pickup including an optical element, and more particularly to an optical pickup which prevents deterioration of track error signals ascribable to adjacent layers when recording or playing data recorded on an optical recording medium having multiple recording layers, and an optical data processing device using the optical pickup.
2. Discussion of the Background
An optical pickup typically has a structure which detects focus error signals and track error signals and controls the position of an objective lens by using these error signals to correctly irradiate a predetermined recording track in an optical recording medium. With regard to the detection of track error signals, a 3-spot system, a push-pull system, and a differential push-pull system (hereinafter referred to as the DPP system or simply DPP) are typical well known examples.
In particular, the DPP system uses a relatively simple optical system with highly sensitive track error signal detection. In addition, the DPP system has an advantage in that it can detect relatively reliable track error signals in which any offset of the track error signals ascribable to displacement of the objective lens or tilt of the optical recording medium is suitably removed.
Track error signal detection using the DPP system is briefly described. An optical pickup which employs the DPP system includes, for example, a diffraction element 23 arranged between a semiconductor laser 1 as the light source and a half mirror 23 as illustrated in FIG. 16. The diffraction element 2 includes, for example, straight grooves engraved in the surface thereof at a constant pitch, that is, with a regular, uniform gap between the grooves as illustrated in FIG. 17, and has a function of splitting a light beam emitted from the semiconductor laser 1 by diffraction into at least three light beams, i.e., + or − one dimensional light beam, and zero dimensional light beam.
These three light beams are independently focused by way of the half mirror 23, a collimate lens 4, and an objective lens 9 to form three focus spots 100, 101, and 102 on the signal recording face of an optical recording medium 10 as illustrated in (a) of FIG. 18A. The irradiation positions of these three spots 100, 101, and 102 are adjusted by, for example, controlling rotation of the diffraction element 2 around the optical axis such that an interval δ between centerlines of the irradiate positions in the radial direction of the optical recording medium 10, i.e., the direction perpendicular to a guiding groove 110 provided in a cyclical manner on the recording surface of the optical recording medium 10, is substantially equal to ½ of the pitch TP of the guide groove 110 (hereinafter, this guide groove pitch TP is referred to as track pitch). The reflected light beams from the focus spots 100, 101, and 102 on the optical recording medium 10 reach the objective lens 9, the collimate lens 4 and the half mirror 23 again. A portion of the reflected light beams transits the half mirror 23 and enters a light reception element 12 via a detection lens 11.
The light reception element 12 has reception portions 20a, 20b and 20c, which are three half- or quarter-reception portions. The reflected light beams of the optical recording medium 10 independently strike predetermined reception surfaces of the reception portions 20a, 20b, and 20c to form detection light spots 200, 201 and 202. The photoelectric conversion signals from these reception surfaces are subjected to subtraction treatment by subtractors 50a, 50b, and 50c to detect the track error signals (hereinafter referred to as push-pull signals) by the push-pull signal system.
The detected light spots corresponding to the main focus spot 100 and the sub focus spots 101 and 102 focused on the recording medium 30 are represented by the detected light spots 200, 201 and 202, respectively. The push-pull signals obtained from the detection spots 200, 201 and 202 are represented by Sa, Sb and Sc. From the relative positions of the focus spots 100, 101, and 102 on the optical recording medium 30, it is apparent that the push-pull signals Sa, on the one hand, and Sb and Sc on the other are about 180° out of phase from each other. With regard to the push-pull signals, Sa and Sb, and Sa and Sc, are output with the signal waveforms reversed (Sb and Sc are the same phase). Therefore, when the addition signal of the signals Sb and Sc is subtracted from the signal Sa, the signal component is not negated but on the contrary is amplified.
On the other hand, displacement of the objective lens 9 or tilt of the optical recording medium 10 causes a predetermined offset component in each push-pull signal. This offset component is obviously independent of the focus spot positions on the optical recording medium 10 and occurs to Sa, Sb and Sc with the same polarity. Therefore, the offset components contained in each push-pull signal selectively cancel each other out in the subtraction treatment described above. As a result, only the offset component is completely removed or significantly reduced so that a good track error signal can be detected.
Thus, for example, the push-pull signals Sb and Sc of (b) of FIG. 18 are added by an adder 51 and the signals thereafter are suitably amplified by an amplifier 52 followed by subtraction treatment from the push-pull signal Sa of the main optical spot 100 by a subtractor 53. Therefore, the offset component contained in the push-pull signal Sa is completely removed or significantly reduced, which leads to output of a suitably amplified track error signal.
A double layer optical recording medium having two data recording layers was invented as a device to increase the storage of optical recording media. Such a double layer optical recording medium has a layer (L1) which is close to the light incident face of the double layer optical recording medium and a layer (L2) which is relatively far from the light incident face. Thus, not only a layer positioned on the focus of the objective lens but also the adjacent layer thereto has an impact on a return light beam. This is referred to as cross talk between layers. Ideally, the optical pickup should be free from effects of cross-talk between layers on servo signals.
To take a specific examples, FIG. 19 is a diagram illustrating a light path when playing data signals on a double layer optical recording medium. With reference to FIG. 19, a light beam L12 reflected at the layer L2 has a focus positioned ahead of that of a light beam L11 received at the light reception element 12 when playing (reading) data signals on the layer L1 closer to the light incident face. On the other hand, a light beam L21 reflected at the layer L1 has a focus positioned behind that of the light beam L11 received at the light reception element 12 when playing data signals on the layer L2.
(a) of FIG. 20 is a diagram illustrating light amount distribution focused on the light reception element when playing data signals on the L1 layer. (b) of FIG. 20 is a diagram illustrating light amount distribution focused on the light reception element when playing data signals on the L2 layer. In (a) of FIG. 20, L11_zero dimensional light beam, L11_+ or − one dimensional light beam, and L12_zero dimensional light beam represent a zero dimensional light beam reflected at the L1 layer, + or − one dimensional light beam reflected at the L1 layer, and a zero dimensional light beam reflected at the L2 layer, respectively, when playing data signals on the L1 layer. In (b) of FIG. 20, L22_zero dimensional light beam, L22_+ or − one dimensional light beam, L21_zero dimensional light beam represent a zero dimensional light beam reflected at the L2 layer, + or − one dimensional light reflected at the L2 layer, and a zero dimensional light beam reflected at the L1 layer, respectively, when playing data signals on the L2 layer.
Generally, a diffraction element for DPP diffracts a light beam with a ratio of zero dimensional light beam, + one dimensional beam, and − one dimensional beam of 10:1:1 with regard to the amount of light diffracted. The interfering light beams from the adjacent layer, i.e., L12_zero dimensional light beam and the L21_zero dimensional light beam, which overlap with the L11_+ or − one dimensional light and the L22_+ or − one dimensional light beam, exceeds an ignorable level in terms of the amount of light. As a result, L12_zero dimensional light beam has an impact on the DPP signal created by the L11_zero dimensional light beam and L_+ or − one dimensional light beam. Similarly, L21_zero dimensional light beam has an impact on the DPP signal created by the L22_zero dimensional light beam and L22_+ or − one dimensional light beam.
In particular, when L12_zero dimensional light beam and the L21_zero dimensional light beam are caused to vary due to variation in thickness between layers, a phenomenon in which track error signals vary arises. As a resulta, precise tracking servo is not possible.
A technology to reduce the occurrence of such cross talk between layers is known in which an optical element is provided to reduce the amount of interfering light beams ascribable to adjacent layers received at the light reception element. FIG. 21 is a diagram illustrating the structure of such an optical system.
The light beam emitted from a semiconductor 1 is focused on the target recording layer of an optical recording medium 10 by a objective lens 9 by way of a diffraction element 2 which diffracts an incident light beam into three beams. The reflected light beam from the target recording layer is detected via the objective lens 9 again by a light reception element 12. In the middle of the course of the light beam, there are provided an outward path to the optical recording medium 10 and a beam splitter 3 which splits the reflected light from the optical recording medium 10 to the light reception element 12. Also, an optical element 16 which reduces the amount of an interfering light beam caused by an adjacent layer received at the light reception element 12 is provided between the beam splitter 3 and the light reception element 12.
The optical element 16 is an element which imparts a phase step to prevent a light beam reflected at the adjacent layer from entering into the light reception element 12 for the optical recording medium 10 (where the optical recording medium has multiple recording layers on one side). (a) of FIG. 22 is a diagram illustrating a front view from the incident light side, and (b) of FIG. 22 is a diagram illustrating a cross section of the optical element 16. As illustrated in (a) of FIG. 22, the optical element 16 is formed of 6 areas of a, b, c, d, e, and f and the areas of b, c and e have a phase step imparting a phase difference of π to the areas of a, d and f.
In addition, (a) and (b) of FIG. 23 are diagrams illustrating a light amount distribution on the light reception element with and without the optical element 16. (a) of FIG. 23 represents the light amount distribution focused on the light reception element when no optical element is provided and is equal to (a) and (b) of FIG. 20. On the other hand, (b) of FIG. 23 represents the light amount distribution focused on the light reception element when the optical element 16 is provided. Each spot of the L11_zero dimensional light beam, the L11_+ or − one dimensional light beam and the L12_zero dimensional light beam is separated into six beams. Therefore, the L12_zero dimensional light beam does not enter into the light reception element and thus the cross talk between layers is avoided. The L11_zero dimensional light beam and the L11_+ or − one dimensional light beam of the signal light are also divided into six. However, this does not have an impact since the light amount received on each element is the same as when the optical element is not inserted.
The objective lens for use in an optical pickup is located on an actuator and movable in the focus direction and the track direction. When the objective lens 9 moves in the track direction, the objective lens 9 is out of position in the track direction with regard to the optical axis of a light beam from the light source as described in FIG. 24 (referred to as optical axis shift). When optical axis shift occurs, a flare light beam interferes as illustrated in FIG. 25. In addition, in terms of the layout of the optical system, the optical element 16 is preferably arranged in the light path commonly shared with the outward path and the return path in some cases.
To avoid this, the optical element 16 is installed on an actuator and moves with the objective lens 9. However, there arises a problem in that the optical element 16 imparts a phase step to the outward light beam advancing from the light source toward the optical recording medium, which degrades the light focus property of the light spot for the optical recording medium 10.